The present invention relates to a system having the function for inputting, storing and retrieving images including color information such as color documents and the function for outputting such information to a display or printer and more specifically to an image processing system for effectively encoding images including monochro images and color images and displaying at a high speed the color images and monochro images during retrieval of information.
For storage and communication of image information, data has been encoded for suppressing redundancy and reducing quantity of information.
Meanwhile, in the case of an apparatus which processes color images as digital data, an image is input as the three kinds of multivalue (level) data generated by color decomposition to the three primary colors of red, green and blue (hereinafter, each element is called respectively as R, G, B and each color element data is written as R data, G data and B data). The color image is then output as the multivalue data or the input multivalue data is binarized and such binary data is output.
For example, a color duplicator directly outputs the input R, G and B multivalue image data. Meanwhile in case the binarization processing such as output to the printer is required, the multivalue RGB data are independently binarized for each color and are then output respectively. The known art which realizes improvement in function of this system is disclosed, for example, by the Japanese Patent Laid-open No. 63-174472. This known art teaches the system for calculating the luminousness information from R, G, B and changes the binarization system depending on the value of such information.
In general, it is desirable, for output of color images, to directly use multivalue data in order to maintain the quality of original images. However, in this case, quantity of data increases and the apparatus for processing data is also complicated. Moreover, an output apparatus such as a printer generally deals with the binary data and therefore the multivalue data must be binarized. However, in case the image data is processed as the binary data, the quantity of data is reduced and the data to be processed is also reduced in amount but image quality may be deteriorated.
In addition, the RGB data has a high correlation between three colors. Particularly, in the expression of monochro image, the R, G and B data have the equal value with each other. Accordingly, redundancy becomes high and encoding efficiency is lowered. The the part expressed as the monochro image in the image is called the monochro domain.
In the case of a color document, the mochchro domain occupies a greader part of text domain. Particularly, it is often seen that red or blue stamps are applied to the monochro document or document is corrected by red characters. These documents are formed by the monochro domain in its greater part.
Therefore, the system processing in direct the multivalue data of R, G and B is not suitable for electronic file system storing a great number of document and image data or facsimile (FAX) which is required to send document and image data at a low data transfer speed because the encoding efficiency for monochro domain is low.
Meanwhile, in TV or VTR, the system, in which the RGB color image data is converted into the luminousness information (hereinafter referred to as Y) and two kinds of chrominance information (hereinafter referred to as I and Q), has been widely employed and an example of using this system for document is also disclosed in the Japanese Patent Laid-open No. 63-9282.
However, the apparatus which conducts conversion to luminousness/chrominance carries out information compression by employing the encoding system for multivalue data such as the orthogonal conversion. However, when the encoding system for multivalue data is employed to document image, following problems rise.
(a) The encoding efficiency is remarkably lowered because concentration change of line and figure such as character is sharp. PA1 (b) Direct output of signal is impossible for the apparatus which processes document as the binary image such as FAX and electronic file system. PA1 (c) Compatibility of data with the conventional monochro system is deteriorated.